C and O Isotope Composition of the Neoproterozoic Pre-, Syn-, and Postglacial Carbonates: Evidence from the Longdor Uplift and Western Slope of the Aldan Shield, Southern Siberian Platform

Abstract

New data on the C and O isotope composition in the Neoproterozoic carbonates of the Ballaganakh and Dal’nyaya Taiga regional stages (southeastern margin of the Patom paleobasin (hereafter, Basin) are presented. The negative δ¹³С anomaly, established in the upper part of the Ballaganakh regional stage, is comparable (in amplitude and stratigraphic position) to the Trezona anomaly, which preceded the Marinoan glaciation. The carbonate layers in glacial deposits of the Nichatka Formation of the Dal’nyaya Taiga regional stage have moderately positive δ¹³С values. The dolomite unit at the postglacial sequence base is characterized by moderately negative δ¹³С values that are typical for the cap carbonate sequence associated with the end-Marinoan glaciation. The lower part of the postglacial sequence (Barakun Formation) is characterized by a gradual increase of calcite in the carbonate component of rocks and a shift in the isotope composition corresponding to the difference in the fractionation coefficient of stable C and O isotopes for dolomite and calcite. This phenomenon, recorded in the postglacial sequences of Namibia and Canada as well, indicates that the carbonate mineral composition was governed by a global change in water chemistry rather than postsedimentary alteration. The overlying carbonate rocks of the Dal’nyaya Taiga regional stage in the epicontinental part of the Patom Basin (Sen Formation) are depleted in ¹³С compared to their shelf analogs in the Ura Uplift. Probably, the carbonates were accumulated asynchronously in the epicontinental and shelf facies of the Patom Basin. The reconstructed trend of variations in the carbon isotope composition for the Dal’nyaya Taiga regional stage is characterized by positive and low-amplitude negative δ¹³С anomalies similar to those in the Doushantuo Formation (China).